The objective of this Phase I application is to develop a new quantitative clinical biomarker for subarachnoid hemorrhage (SAH). Acute headache is initially screened for SAH by computed tomography (CT). CT-negative patients suspected of SAH are then screened for cerebrspinal fluid (CSF) xanthochromia, which has documented limitations and is not FDA-approved. The present application proposes to develop a new biomarker, cleaved MAP-tau (C-tau). The proposed biomarker should demonstrate increased sensitivity and specificity, as well as, improving point of care treatment by using immunocard technology. We have developed a sandwich ELISA that specifically quantifies this neuronally localized protein, C-tau, that is released from damaged neurons. Employing our C-tau ELISA, we demonstrate in preliminary studies that CSF C-tau levels are elevated 1 ,000 fold in SAH patients compared to neurologic controls. Our Specific Aims are: Specific Aim 1: Determine if CSF C-tau levels are significantly elevated in CT-negative acute headache patients with SAH compared patients without SAH determined at three month follow up (N=150). Specific Aim 2: Determine if differences in CSF C-tau level between SAH(+) and SAH(-) patients are time dependent (Specific Aim 1).